Plastics additives are additions to polymers which have a modifying and/or stabilizing action on the polymers. In addition to a chemical part structure in the molecule which is decisive for their function, as a general rule they have substituents with which the xe2x80x9ccompatibilityxe2x80x9d (for example solubility, diffusion properties, resistance to extraction and the like) of the additive with the polymeric matrix is regulated (literature: Gxc3xa4chter/Mxc3xcller, Kunststoffadditive [Plastics Additives], 3rd edition 1990, Carl Hanser Verlag Munich, Vienna, page 33 et. seq.). The functional part of the molecule is also often regulated in its reactivity by very bulky chemical substituents (xe2x80x9csterically hindered phenolsxe2x80x9d as antioxidants, xe2x80x9csterically hindered aminesxe2x80x9d as light stabilizers).
These structural elements have the effect that crystallization of such additives during preparation often proceeds only with a delay and/or is incomplete and/or leads to several crystal modifications which can easily be converted into one another. This leads to wide melting ranges (instead of sharp crystallite melting points) or multiple melting points which merge into one another.
Immediately after preparation of a pulverulent handling form, this delayed crystallization and/or rearrangement of the less stable into more stable crystallite structures means that during storage and handling of the products in the course of time, crystallizates grow into the surfaces of particles in contact with them, which leads to mechanical adhesion, felting or caking of the material. The lumps formed in this way interfere considerably in the flow properties and the meterability of the additives, which makes the additive unsuitable for its use. The interference is particularly pronounced if the melting range is below 150xc2x0 C.
Numerous attempts have been made to improve this property, for example the addition of highly disperse silicic acids or of metal stearates or oxides have been recommended, to powder the surface of the particles which tend to stick (literature: for example Degussa brochures from the pigments publication series: xe2x80x9cAerosil zur Verbesserung des Fliessverhaltens pulverfxc3x6rmiger Substanzenxe2x80x9d [Aerosil for improving the flow properties of pulverulent substances], no. 31, 2nd edition 5/1978 and xe2x80x9cSynthetische Kieselsxc3xa4ure als Fliexcex2hilfsmittel und als Trxc3xa4gersubstanzxe2x80x9d, [Synthetic silicic acid as a flow auxiliary and as a carrier substance], No. 31, 5th Edition, 5/1992]. One disadvantage of this method is the introduction of undesirable foreign substances into the actual additive.
The preparation of tablet-like, formulations of powder applying a high pressure has also been promoted. The additional working step, which makes the product more expensive and involves a risk of contamination due to mechanical abrasion, must be regarded as a serious disadvantage here. Solidification of the melt and subsequent grinding is also practised. A disadvantage here is that the end product felts and cakes readily under pressure (for example under the weight of a stack of pallets during transportation in summer) and has a high dust content. Spray cooling from the melt has also been proposed, but this involves the disadvantage of very poor control of the crystallization and often very pronounced tendency to caking.
U.S. Pat. No. 4,578,021 describes a machine, with the aid of which granules of uniform size can be prepared, so that subsequent grinding becomes superfluous, from a two-phase mixture comprising a melt and crystal seeds via a drop former with subsequent cooling. The formation of deposits and encrustations in the drop former customary to date is avoided in this process.
The object of the present invention was to provide additives for plastics which, without addition of foreign substances, are still free-flowing and readily meterable even after relatively long storage and handling.
It has now been found that the tendency of such additives to cake can be eliminated, the meterability and the flow and transportation properties can be improved considerably and the formation of dust or abraded material can be greatly reduced by plastics additives by cooling a mixture of 99.9 to 10% by weight of molten additive material and 0.1 to 90% by weight of crystalline additive material after a shaping process.
By changing the content of crystalline substance, the viscosity of the mixture can be varied within wide limits and adapted to the requirements of the shaping process. The end product is formed here in its stable crystal modification and undergoes no further rearrangement leading to felting of the particles. The end product is formed in a defined, uniform particle size (for example from 0.1 to 10 mm) which can be adjusted in a controlled manner within wide limits by varying the melt viscosity and the shaping process.
The plastics additives according to the invention can advantageously be prepared by employing a mixture of 9.5 to 25% by weight of molten additive material and 0.5 to 75% by weight of crystalline additive material.
Plastics additives prepared by employing a mixture of 90 to 30% by weight of molten additive material and 10 to 70% by weight of crystalline additive material are also particularly suitable.
Plastics additives which are prepared by employing a mixture of 70 to 50% by weight of molten additive material and 30 to 50% by weight of crystalline additive material are particularly preferred.
The rapid cooling of the mixture can preferably be carried out on a cooled conveyor belt.
Forcing the mixture out of a tool provided with die openings (perforated die) has proved to be a particularly suitable shaping process.
Shaping processes which can be employed are, for example:
forcing the mixture out of a cylindrical die arrangement by means of a choker bar
forcing the mixture out of a cylindrical die arrangement with internal toothing and a die bore in the tooth base by an internal toothed wheel
forcing the mixture out of a cylindrical die arrangement with internal bores and a die bore in the bore base by a stamp
forcing the mixture through a flat perforated plate by means of edge runners
forcing the mixture through a flat perforated plate by means of an extruder
passing through a perforated plate by rotating rolls
extrusion with an extruder (single- or twin-screw extruder)
pressing out through a mouthpiece of a stapling press provided with several die openings
pressing out through a pipe die provided with numerous die openings by means of a stripper doctor blade
As a result of the content of crystalline he ten material, easier temperature regulation is achieved, since on cooling, the crystals which increasingly separate out counteract the reduction in temperature by their heat of crystallization, and vice versa.
Plastics additives on which the process according to the invention can be used successfully are, inter alia, additives from the class of UV stabilizers, antioxidants, heat stabilizers, lubricants, antistatics, adhesion promoters, agents which impart compatibility, dispersing auxiliaries, acid scavengers, metal deactivators, peroxide scavengers, processing stabilizers, nucleating agents, optical brighteners, slip agents, antidew agents and flameproofing agents.
Particularly suitable examples are, inter alia, 2-hydroxy4-n-octoxybenzophenone, lauric acid diethanolamide, glycerol mono-, di- and tristearate and mixtures of these glycerol stearates, dioctadecyl sulfide, mixtures of dialkyl mono-, di- and polysulfides, distearyl thiodipropionate, dilauryl thiodipropionate, bisstearoylethylenediamine, pentaerythrityl tetrakis-3-(3,5-di-tert-butyl4-hydroxyphenyl)propionate, octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, tetrakis-(2,4-di-tert-butyl-phenyl) 4,4xe2x80x2-biphenylenediphosphonite, distearyl pentaerythritol diphosphite, di(2,2,6,6-tetramethylpiperidin4-yl) sebacate and bis(2,4-di-tert-butylphenyl) pentaerythritol disphosphite.